"wretched class they thrust into unwholesome dungeons, to live or die, they cared not which. They have deprived them of the liberty and free air which heaven has never denied them, and then left them to suffer for food and clothing. In view of all this, a national fast is proclaimed! Oh, what an insult to Jehovah! The Lord saith by the mouth of Isaiah: “Yet they seek Me daily, and delight to know My ways, as a nation that did righteousness, and forsook not the ordinance of their God.” 1T 257.1"
"Is not this the fast that I have chosen? to loose the bands of wickedness, to undo the heavy burdens, and to let the oppressed go free, and that ye break every yoke?"
The EGW passage directly quotes Isaiah 58:3 (though she cites it as Isaiah without the chapter). The context of the EGW passage is about the hypocrisy of proclaiming a national fast while oppressing people, which is the exact point Isaiah 58 makes. The connection is direct and intentional.